Proprietary Attribute

The nosave attribute was originally intended to signal to browsers that you did not want users to be able to right-click and download your image. It was never a part of the HTML specification, and was only ever implemented in a handful of browsers.

Use JavaScript

You can achieve something similar with the inline JavaScript attribute oncontextmenu.

Or maybe don’t bother

Remember: Even if you do this, it is rather easy to circumvent. Any content, image, data, or text that makes it onto a user’s browser is — one way or another — exposed for the user to inspect. All this sort of thing does is annoy users and feed your own paranoia about “people stealing my content.” Most people don’t want to steal your content. The people who do want to steal it will find a way to steal it anyway. Spend your web design energy on somethingmore productive.